mokk.me
Easily create mobile web app mocks you can share, test on multiple platforms, and use in production. Beta. Really. Incomplete and contains bugs. But have fun already.
iPlotz
… wireframing, mockups and prototyping for websites and applications
Mockingbird
Wireframes on the fly … an online tool that makes it easy for you to create, link together, preview, and share mockups of your website or application.